From: Mark Woodward <markonlinux@...>
Date: 2007-01-03T20:15:06+09:00
Subject: Array questions

Hi,

I'm trying to learn ruby by 'borrowing' peoples code and adding comments
to help me understand what they've done. The code below is 'borrowed' from
the maze solutions on rubygarden.
(http://wiki.rubygarden.org/Ruby/page/show/NubyMazeSolutions)

I'm trying to understand the line:
    @paths = Array.new(0,Array.new(0))

From 'ri Array.new' I think the outer Array is of the form:
    Array.new(size=0, obj=nil)
    where obj=nil is Array.new(0)   ???

If so could someone explain whats going on here? I can understand
something like Array.new(5,"A") where a new Array with 5 references to the
same string "A" is created. But why create an Array with 0 elements and
have 0 references to another new empty array? How can you have 0
references?

Aren't these equivalent?

irb(main):034:0> Array.new(0,Array.new(0))
=> []
irb(main):035:0> Array.new(0)
=> []
irb(main):036:0> Array.new
=> []

I can replace that line in the code below with any of the 2 commented
lines below it and the program still runs. So I'm wondering why the author
used that particular line. I understand no one can speak for the author
but am querying whether I'm missing something here.


--------------------------------------------------------------------------
class MazeSolver


 # the constructor with the maze string as a parameter
  def initialize(m)

    # assign the maze to the instance variable @maze
    # (only visible to a particular instance of the class
    # as opposed to a class variable (@@) which is accessible 
    # to *any* instance of the class)
    @maze = m

    # find the length of the first line. ie count chars to first \n
    # (indexing starts at 0 so + 1)
    @rowlen = @maze.index("\n")+1

    # create an array of size 0, ?? 
    # with 0 references to another new empty array ?????
    # hah?
    @paths = Array.new(0,Array.new(0)) 			<<<<<<<<<<<<
    # @paths = Array.new
    # @paths = Array.new(0)

    # call the findPaths method
    findPaths
  end


  # 
  def getNSEW(index)

    # create an empty array
    ret=[]

    # 
    table =[index-@rowlen,index+@rowlen,index+1,index-1]
    table.each{|n|ret.push([n,@maze[n].chr])}
    return ret
  end


  def findPaths(loc=@maze.index('s'),current=[])
    sides = getNSEW(loc)
    return 0 if current.include?(loc)
    current.push(loc)
    ret=0
    (0..3).each do |n|
      case (sides[n][1])
      when "e": current.push(sides[n][0]);@paths.push(current);return 1
      when "-": ret += findPaths(sides[n][0],current.dup)
      end
      n+=1
    end
    ret
  end


  def to_s
    ret=""
    @paths.each do |path|
      tMaze = @maze.dup
      path.each{|loc|tMaze[loc]="*"}
      ret += "#{tMaze.gsub(/#/,"I").gsub(/-/," ")}\n\n\n"
    end
    ret
  end


end


# a heredoc created string representing the maze
mazeDef = <<MAZE_STRING
#########################
#s--###----#####-#####-##
###-#---##-##--#-##-----#
###-#-#-#--##-##-##-###-#
#---#-#-#-###-##-##-##--#
#-#---#-#---#-##----##-##
#-#-#-#-#-#-#-##-########
#-#-#-#-#-#---##--------#
#####-###---###########-#
#####-#####-###########-#
#####------------------e#
#########################
MAZE_STRING


# 'print' calls the to_s method of a newly created MazeSolver
# instance where mazedef is passed to the initialize method.
print MazeSolver.new(mazeDef)

--------------------------------------------------------------------------


thanks,


-- 
Mark